The .h file shouldn't include the .cpp file. The .h file usually contains only declarations. (In this case, declaration of your stripos function.) Any .cpp file that wants to use the functions (and the single .cpp that defines those function) should include the header file. Like so:

// the following function definition should match the declaration in the header
// if it doesn't, and other code tries to use the function as declared,
// you may get "symbol not defined" errors from the linker
int stripos (const string s , const string find, const int offset)
{
if ( offset >= (int)s.length())
{
return false;
}
// .. etc ..
}

Also, the name "strings.h" makes me nervous. It's hard to imagine a more common name for a header file! I don't know what else you are planning to put it in but I would call it stringutils.h.